1. Field of the Invention
This invention relates to a heat vulcanizable silicone rubber composition which strongly adheres to a wide variety of substrates. This invention also covers a composition useful as an additive for heat activated organic peroxide vulcanizable rubber to provide the bonding characteristic to the rubber; and the silicone rubber compositions containing the additive and methods of using.
2. Description of the Prior Art
Silicone rubber has become one of the useful specialty elastomers available to industry. In many applications it is desirable to bond silicone rubber to various substrates. Silicone rubber does not normally adhere to solid surfaces as strongly as desired. Over the years, various methods have been devised to improve the bonding of silicone rubber to different types of substrates. These methods include the use of a primer on the surface of the substrate, as well as additives to the unvulcanized silicone rubber.
In U.S. Pat. No. 2,744,878, issued May 8, 1956, Smith-Johannsen discloses elastic organopolysiloxanes of improved heat resistance and adhesion containing an organic silicate and calcium carbonate. His composition comprises an organopolysiloxane, calcium carbonate filler, peroxide curing agent, and liquid partial hydrolysis product of monomeric organosilicates. The organosilicate is said to improve the adhesion to surfaces. He teaches his composition must be used and cured within 12 hours after mixing, unless the filler and organosilicate are heated together before the addition of the other ingredients.
In U.S Pat. No. 2,751,314, issued June 19, 1956, to Keil, it was noted that methods of bonding some few silicone rubbers to some few solid materials was known, but no single means for bonding many combinations was known.
Keil teaches a method of adhering silicone rubber to the surface of a solid comprising first depositing a titanium compound coating on the surface, then depositing on the titanium coating a coating consisting essentially of a mixture of partially hydrolyzed alkylpolysilicate and toluene soluble organopolysiloxanes. If desired, additional layers of silicone rubber can be deposited. The method is stated to provide adhesion to many types of surfaces. In his Example 4, he showed that a room temperature vulcanizing rubber prepared from dimethylsiloxane polymer, diatomaceous earth, ethylpolysilicate, and lead octoate did not adhere to an aluminum panel without his primer system. In Example 5, he states that equivalent results are obtained on steel, cadmium steel, nickel, tin, copper, porcelain, resin laminates and glass cloth.
In U.S. Pat. No. 3,261,739, issued July 19, 1966, Porter teaches a laminated glass windshield containing as an interlayer, a layer of transparent organosilicon elastomer. In the preferred embodiment, the interlayer comprises an organopolysiloxane gum, filler, ethylpolysilicate and dicumyl peroxide catalyst. The cured elastomer was said to have excellent adhesion to glass, electrically conducting films deposited on glass and polyvinylbutyral, but there is no indication of the actual bond strength obtained.
Smith-Johannsen in U.S. Pat. No. 3,325,440, issued June 13, 1967, teaches the addition of unhydrolyzed organosilicates to curable organopolysiloxanes to improve the reversion resistant at high temperatures. Ethylorthosilicate is shown to be useful in his system. When his procedure is followed, the composition is stated to adhere to a variety of substrates, but no indication is given as to the degree or type of adhesion achieved.
Foster in U.S. Pat. No. 3,379,607, issued Apr. 23, 1968, teaches that bonding of cured diorganosiloxanes to solid substrates is best accomplished through the use of a primer system. Foster teaches that adding a compound, such as methacryloxypropyltrimethoxysilane, to the uncured elastomer as a rule possesses the disadvantage of inducing physical property changes in the polysiloxane which are undesirable. Foster shows a primer obtained by contacting a phosphorus containing compound with a silane containing at least one CH.sub.2 .dbd.CH group. Foster obtained cohesive failure, at least in some cases, when using his primer system.
DeZuba et al. in U.S. Pat. No. 3,730,932, issued May 1, 1973, teach a self-bonding, heat-curable composition that includes a maleate or silylmaleate as an additive to promote bonding to a substrate. In U.S. Pat. No. 3,813,364, issued May 28, 1974, they show that cyanurates, isocyanurates, silyl-cyanurates, and silylisocyanurates are also useful.
In U.S. Pat. No. 3,794,612, issued Feb. 26, 1974, Plueddemann teaches a silicone rubber composition having improved adhesion to unprimed substrates. His composition comprises a silicone rubber stock and 2 to 10 parts of diallylphthalate. Pleuddemann theorized that the diallylphthalate functions because it is relatively compatible with the unvulcanized silicone rubber, but separates because it is only partially compatible during vulcanization and thus provides a surface layer having adhesive properties. The mixture is further enhanced by the addition of an alkoxy silane having a vinyl or allyl radical. A mixture containing diallyl o-phthalate and vinyltrimethoxysilane adhered to glass and aluminum, giving cohesive failure of the rubber when tested after 4 hours in boiling water.
Mine et al. in U.S. Pat. No. 4,033,924, issued July 5, 1977, point out in their discussion of the prior art that curable silicone rubbers and resins which cure through the application of heat and peroxide catalyst do not possess completely satisfactory adhesion to other materials. They also point out the disadvantages of using primer systems. They then teach a composition which consists of an organopolysiloxane, filler, organic peroxide catalyst, and an organosilicon compound containing at least one epoxy group, at least one alkyl group, and at least one low molecular weight alkenyl group or hydrogen atom bound to silicon. The composition, when cured in contact with a surface, adheres to metals, plastics, ceramics, fibers, wood, paper and others. Their examples show cohesive failure on bonding to ceramic surfaces and plastic surfaces, while improved peel adhesion was shown for various metal surfaces. Without the epoxy containing additive, the compositions did not bond sufficiently to give cohesive failure; the adhesive failure was at much lower values.
Smith in U.S. Pat. No, 4,177,301, issued Dec. 12, 1979, then shows a primer system suitable for bonding both room temperature vulcanizing silicone rubber and heat vulcanizable silicone rubber. Smith teaches that self-bonding, heat vulcanizable silicone rubber compositions, which he has previously mentioned as incorporating a silyl isocyanurate or silylmaleate as a self-bonding additive, have difficulties. He points out that they are usually a very complex type of system and they require the addition or presence in the composition of very complex chemical self-bonding additives, which unduly increase the cost of the composition. He further states that without the use of a primer such compositions do not have as high an adherence to the substrate as would be desired in some cases, as when the cured silicone elastomer is under water immersion for a substantial period of time. Smith teaches the use of a primer produced by reacting an alkylacrylate or alkylmethacrylate with an acrylate silane then adding additional acrylate silane.
Belgian Pat. No. 878,521, issued Feb. 29, 1980 to Murray, teaches a method for improving bonding of a silicone rubber to a substrate surface. The method mixes silicone rubber base, optional extending filler, organic peroxide vulcanizing agent, and from 0.1 to 1.5 parts by weight of an acryloxyalkylsilane. The mixture is formed against a surface and heated to vulcanize, yielding a vulcanized silicone rubber bonded to the substrate surface. The composition is shown to bond to aluminum, cold-rolled steel, stainless steel, and glass fiber fabric.
U.S. Pat. No. 4,202,812, issued May 13, 1980 to Murray, teaches silicone rubber compositions containing extending fillers and acryloxyalkylsilanes. He states that at 0.5 part of acryloxyalkylsilane per 100 parts of silicone rubber base, the cured composition can adhere to the aluminum plates used in the molding process.
In spite of the vast teaching of the art, much of it conflicting, there is still need to improve the bonding capabilities of heat vulcanizable silicone rubber compositions to substrates. Many of the systems taught provide only adhesive type bonding. Such a type of bonding is not satisfactory in most cases. The systems using a primer require a separate step in the bonding process which increases costs. In many cases the primers must be carefully applied and cured in order to function properly. The primed surface must be adhered to within a short period of time in order to obtain the best results. Many of the systems using additives have disadvantages such as failure to form bonds with a wide variety of substrates, failure to maintain satisfactory bond during use conditions such as exposure to elevated temperature or water immersion, and failure to form satisfactory bonds when used in conjunction with many kinds of heat vulcanized silicone rubber and vinyl specific or non-vinyl specific organic peroxide vulcanizing agents.
Satisfactory bonding between heat vulcanizable silicone rubber and a variety of substrates is very complex, particularly when an additive approach is used. The additive must provide all the advantages of improved bonding without adversely reacting with the silicone rubber to degrade the rubber properties.
In order for a bonding system to be completely satisfactory, the bond between the rubber and the substrate should be at least as strong as the rubber itself. When the rubber is forced away from the substrate, the failure should be by cohesive failure of the rubber. Such cohesive failure should occur with the widest possible variety of substrates, many kinds of silicone rubber, and many organic peroxide catalysts.